Development of downflow hanging sponge (DHS) reactor as post treatment of existing combined anaerobic tank treating natural rubber processing wastewater

Conventional aerated tank technology is widely applied for post treatment of natural rubber processing wastewater in Southeast Asia; however, a long hydraulic retention time (HRT) is required and the effluent standards are exceeded. In this study, a downflow hanging sponge (DHS) reactor was installed as post treatment of anaerobic tank effluent in a natural rubber factory in South Vietnam and the process performance was evaluated. The DHS reactor demonstrated removal efficiencies of 64.2 +/- 7.5% and 55.3 +/- 19.2% for total chemical oxygen demand (COD) and total nitrogen, respectively, with an organic loading rate of 0.97 +/- 0.03 kg-COD m(-3) day(-1) and a nitrogen loading rate of 0.57 +/- 0.21 kg-N m(-3) day(-1). 16S rRNA gene sequencing analysis of the sludge retained in the DHS also corresponded to the result of reactor performance, and both nitrifying and denitrifying bacteria were detected in the sponge carrier. In addition, anammox bacteria was found in the retained sludge. The DHS reactor reduced the HRT of 30 days to 4.8 h compared with the existing algal tank. This result indicates that the DHS reactor could be an appropriate post treatment for the existing anaerobic tank for natural rubber processing wastewater treatment.